Ideally, any antioxidant for organic substrates such as rubber or other plastic material, whether providing protection in the presence of atmospheric oxygen, ozone or any other oxidant, should not introduce objectionable characteristics into the substrate which make it hazardous to handle, more subject to premature curing or weaker physically. The antioxidant should prevent oxidative attack over extended periods of time without volatilization or change in visual appearance of the organic substrate caused by staining or change in color as by frosting as well as cracking or cutting of the subsrate surface.
Antioxidants for use in any substrate subject to change induced by either chemical or physical action ideally prevent that change for extended time periods without adversely affecting the properties of the substrate. Any composite effect derived from the use of a single antioxidant such as combined heat, light, aging, oxidation and flex cracking stability, extend the general applicability of that antioxidant.
Although attack on carbon-carbon unsaturation by ozone may be generically considered oxidation of the substrate, the intermediate ozonides and/or hydroperoxides present a unique problem which is not solved by antioxidants generally. Thus, antiozonants are a special group of antioxidants which in themselves may or may not be functional toward attack by atmospheric oxygen.
The function of ozone in initiating cracking of rubber has been studied extensively since 1945. Various antiozonants have been discovered which extend the useful life of rubber exposed to ozone by extending the time for initial cracking and retarding the extent of cracking in rubber. Many commercial antiozonants are derived from the N-substituted paraphenylene-diamines.
Those N-substituted para-phenylenediamines prepared by alkylating para-aminodiphenylamine with a ketone such as the 4-isopropylamino diphenylamine are effective antiozonants and do not cause premature curing or retard the curing rate when used in rubber. However, the 4-alkylaminodiphenylamines are colored and stain the rubber in which they have been incorporated.
Conventionally, antioxidants are introduced into rubber in the latex stage. A solid antioxidant is dispersed in the latex to provide a dispersion approaching as closely as possible a homogeneous mixture while liquid antioxidants are mixed with the latex by emulsification. At this point, it is very important that the antioxidant not affect premature coagulation of the rubber or cause creaming of the latex (rubber destabilization and separation from water in the latex). After introduction of the antioxidant, the rubber is coagulated to separate it from the water as an elastomeric dry material. The rubber appears as small agglomerated masses which are dried by hot air to form crumbs of rubber. Another addition of antioxidant is conventionally made to the crumb or to compressed crumb rubber during milling, extruding or other fabrication treatments.